Tokamak
From Freepedia
A tokamak is a machine producing a toroidal (doughnut-shaped) magnetic field for confining a plasma. It is one of several types of magnetic confinement devices and the leading candidate for producing fusion energy. The term tokamak is a transliteration of the Russian word Токамак which itself comes from the Russian words: "тороидальная камера в магнитных катушках" (toroidal chamber in magnetic coils). It was invented in the 1950s by Igor Yevgenyevich Tamm and Andrei Sakharov.
The tokamak is characterized by azimuthal (rotational) symmetry and the use of the plasma current to generate the helical component of the magnetic field necessary for stable equilibrium. This can be contrasted to another toroidal magnetic confinement device, the stellarator, which has a discrete (e.g. five-fold) rotational symmetry and in which all of the confining magnetic fields are produced by external coils with a negligible current flowing through the plasma.
Why doughnut shaped?
The distinctive shape of the fusion reactor is necessary in order to produce a magnetic field with as few irregularities as possible. The doughnut has a particular topological property that a sphere (for example) does not have. The problem is referred to as the hairy ball theorem. Imagine a sphere with hair growing out of it. The hair is analagous to the magnetic field lines needed in a fusion reactor. It turns out that it is impossible to comb hair on a sphere so that no hair sticks up. A strand of hair that is standing on end would be an instability in the reactor. However, a hairy doughnut can be so combed, and thus adjustments to the magnetic field can be made to correct the irregularities. This allows the magnetic field to better confine the plasma.
See also
- Tokamak experiments
- The section on Dimensionless parameters in tokamaks in the article on Plasma_scaling
External links
- Plasma Science - excellent site on tokamaks.
- Fusion programs at General Atomics, including the DIII-D National Fusion Facility, an experimental tokamak.
